A chair generally contains a subframe and a seat part supported thereon and having a seat surface and a backrest which is disposed in the region of an edge of the seat surface at least approximately perpendicularly to the seat surface, and a contact surface for the back of a person sitting on the chair.
In order to permit comfortable movements of the upper body of a person sitting on the chair, various mechanisms are provided for the backrest so that this backrest follows movements of the upper body within a preset extent. Mechanisms which permit backwards movement of the backrest against a return force are widely used especially in office chairs. Furthermore, a chair has become known in practice which has a backrest which can elastically deflect by means of an elastic element such as e.g. a rubber joint.
In chairs, a balance always has to be found between stability and support function on the one hand and comfort on the other. In particular, when sitting on a chair for long period as required by work, a chair in which the backrest also performs each movement of a person sitting thereon with or without a counterforce is no more optimal than a chair with a backrest which is positioned in an entirely rigid manner. Furthermore, complex mechanisms for backrest moveability make chairs extremely expensive so that many users do not buy or cannot afford such chairs.